1. Field of the Invention
The invention relates generally to the field of electromagnetic survey apparatus for subsurface exploration in the Earth. More particularly, the invention relates to structures for detector electrodes and arrays thereof for detection of electric fields resulting from electromagnetic fields imparted into the Earth.
2. Background Art
Electromagnetic surveying is used for, among other purposes, determining the presence of hydrocarbon bearing structures in the Earth's subsurface. Electromagnetic surveying includes what are called “controlled source” survey techniques. Controlled source electromagnetic surveying techniques include imparting an electric current or a magnetic field into the Earth, when such surveys are conducted on land, or imparting the same into sediments below the water bottom (sea floor) when such surveys are conducted in a marine environment. The techniques include measuring voltages and/or magnetic fields induced in electrodes, antennas and/or magnetometers disposed at the Earth's surface, in the water or on the sea floor. The voltages and/or magnetic fields are induced by interaction of the electromagnetic field caused by the electric current and/or magnetic field imparted into the Earth's subsurface (through the water bottom in marine surveys) with subsurface Earth formations.
One type of marine controlled source electromagnetic surveying known in the art includes imparting alternating electric current into the sediments below the water bottom by applying current from a source, usually disposed on a survey vessel, to a dipole electrode towed by the survey vessel. A dipole electrode is typically an insulated electrical cable having two electrodes thereon at a selected spacing, sometimes 300 to 1000 meters or more. The alternating current has one or more selected frequencies, typically within a range of about 0.1 to 100 Hz. A plurality of detector electrodes is disposed on the water bottom at spaced apart locations, and the detector electrodes are connected to devices that record the voltages induced across various pairs of such electrodes. Such surveying is known as frequency domain controlled source electromagnetic surveying. Frequency domain EM survey techniques are described, for example in U.S. Pat. No. 7,026,819 issued to Eidesmo et al.
Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic surveying. In transient controlled source electromagnetic surveying, electric current is imparted into the Earth's subsurface using electrodes on a cable similar to those explained above as used for frequency domain surveying. The electric current may be direct current (DC). At a selected time or times, the electric current is switched off, and induced voltages are measured, typically with respect to time over a selected time interval, using electrodes disposed on the water bottom as previously explained with reference to frequency domain surveying. Structure and composition of the Earth's subsurface are inferred by the time distribution of the induced voltages. Transient electromagnetic surveying techniques are described, for example, in U.S. Patent Application Publication No. 2006/0186887 filed by Strack et al.
Irrespective of the technique used, the presence of hydrocarbon bearing structures can be inferred because of resistivity contrast between hydrocarbon bearing structures, which can have electrical resistivities in a range of several ohm-meters to several hundred ohm-meters, and those of the adjacent, non hydrocarbon bearing Earth formations, which may have resistivities in a range of about 0.2 ohm-meters to several ohm-meters.
There continues to be a need for improved techniques and apparatus for mapping the Earth's subsurface using electromagnetic measurements.